1. Field of the Invention
The present invention relates to baseband processors in general, and more particularly, to an automatic phase and gain balance controller for separately sensing phase and magnitude errors representative of the unbalance in the baseband processor channels which generate the inphase and quadrature components and nulling these sensed errors by providing balancing adjustments in the channels.
2. Description of the Prior Art
Typically, a baseband processor is utilized by a radar system for the detection of a moving target using doppler processing techniques. Basically, a moving target indicator (MTI) type radar system transmits a series of fixed or variably spaced constant frequency pulses to a target. The pulsed radar signals are reflected from the target(s) and are detected by the radar system which is capable of sampling the detected echo signals at predetermined time intervals after transmissions, more commonly referred to as range cells. The distance or range of the target from the radar system is generally determined as a function of the time required for the propagated pulsed signals to travel to and to be reflected from the target. Subsequent to a transmitted pulse and prior to the next transmitted pulse, that is within the interpulse period, there typically exists a period of time in excess of the time corresponding to the maximum instrumented range of the radar system. This time period is normally referred to as dead time due to the ineffective operating state of the radar system.
Basic to the operation of the doppler processor of the MTI radar system is the variation or shift in frequency imparted to the return echo signals from the moving target. This doppler shift, which is usually measured in the frequency domain by noting the location of the spectral response peak, is used to determine the velocity of a specified target. Normally, the echo signals comprise both desired echo signals from relatively fast-moving targets and unwanted clutter echo signals from relatively slow-moving or stationary objects such as buildings, low-elevation objects, rain and the like which are within the specified range cell of the target.
In a typical MTI radar system employing a coherent radar receivier, a constant transmission frequency is generally derived from a coherent master oscillator, which synchronizes the operation of the radar receiver, and is transmitted in pulse bursts of approximately 0.5 microseconds at intervals generally spaced at around a millisecond. The echo signals are normally conditioned by a conventional RF amplifier and then mixed with a reference frequency also derived from the coherent master oscillator (COHO) to obtain an IF signal generally centered at 30 megahertz wherein the echo signal information remains intact. In most radar applications, it is important to discriminate between wanted targets and unwanted clutter not only on the basis of the speed of a moving object, but also on its direction and relation to the radar system.
Present MTI radar systems employ baseband processors to generate both an in-phase signal and quadrature signal from the IF signal containing the echo signal information to derive both the speed and direction of the moving object. A baseband processor is shown typically in FIG. 1 wherein the IF signal 1 is an input to two synchronous detectors 2 and 3. A reference frequency 4 generally of the same frequency and phase as the center frequency of the IF signal 1 is derived from the COHO 5 and conducted unaltered in phase to the synchronous detector 2, wherein a conventional mixing operation is performed yielding sum and difference frequency components of the signals 1 and 4. A low pass filter (LPF) 6 filters out the sum frequency components of the output signal of the synchronous detector 2 to provide only inphase signal frequencies which are within a narrow band from zero to approximately 2 megahertz, commonly referred to as baseband. Also the reference frequency signal 4 is shifted 90.degree. out-of-phase by a typical phase shifter 7 and the phase shifted signal 8 is provided to the synchronous detector 3 wherein a conventional mixing operation is performed to yield quadrature sum and difference signals. A quadrature sum signal is filtered by a low pass filter (LPF) 10 to yield only quadrature frequency signals with the specified baseband. These baseband frequency in-phase and quadrature analog signals may be digitized by conventional analog-to-digital (A/D) converters 11 and 12, respectively, for further digital processing by the MTI radar system, for example, wherein stationary and moving objects may be discriminated.
Synchronous detectors such as those used in the baseband processor of the radar receiver described above may be subject to phase and amplitude channel unbalance in generating the I and Q signals. In particular, this unbalance distortion may include the generation of spurious (image) frequency components in the doppler spectral response curves derived by the MTI radar system which would not be normally present using ideal synchronous detectors. In addition, the A/D converters described in connection with the baseband processor of FIG. 1 may produce an additional channel amplitude imbalance, contributing further to the signal distortion. Various manual circuit adjustments can be employed to eliminate the unbalance effects, however the practical limitations to the stability of the adjustment settings such as drift as a function of time and ambient conditions (temperature, etc.) may still cause distortion in the doppler spectral response curves which may exceed that allowable for critical conditions.
One case which illustrates the effects of the unbalanced errors of a baseband processor is manifested in an MTI radar system employing a pair of ground and rain clutter canceller filters for cancelling echo signals from ground obstacles and wind-borne rain. Each filter is designed with unsymmetrical doppler responses as depicted in FIGS. 2A and 2B. Typically, wind-borne rain interference may be detected as either being blown in a direction away from or into the antenna of the MTI radar system. One of the pair of digital canceller filters is selected to provide the necessary attenuation stopband, as defined by either frequency +f.sub.1 or -f.sub.1 as shown in FIGS. 2A and 2B, respectively, to suppress the rain echo signal interference corresponding to the direction in which the rain is being blown with respect to the scan of the antenna. It is apparent that the rain echo signal may resemble a moving target and may create false alarm conditions if permitted to pass further into the digital processing of the MTI radar system. However, this condition is alleviated as a result of the attenuation by the pair of filters.
Since it is typically not possible to build a very broad filter which has a wide notch in both the positive and negative doppler frequency regions concurrently, the MTI radar system appears to be vulnerable to spurious image frequency components of the rain clutter echo signals. The power spectral density of typical rain clutter is shown in FIG. 3. The MTI filter response shown in FIG. 2A would be appropriate for attenuating such unwanted rain clutter returns. However, when channel unbalance causes the generation of spurious image frequency components as discussed above and indicated in FIG. 3, it is evident that the MTI filter response shown in FIG. 2A is unable to suppress these spurious image frequency components. These image frequency components will normally fall in the passband of the MTI filter and not be attenuated as exhibited by FIGS. 2A and 3. Further, these image frequency components may resemble targets to the MTI radar system creating possible problems such as false alarms or reduction of radar system sensitivity.
Another example case of the effects of unbalance errors in a baseband processor is manifested in a radar doppler filter with a relatively narrow passband, not centered at zero frequency, and with low doppler sidelobes at the passband image frequency. With ideal phase-detector and A/D converter circuits, any signals falling in the passband image frequency range will be heavily attenuated by the low doppler sidelobes. However, synchronous detector and/or A/D converter unbalance may produce distortion components of the echo signals at their image frequencies, which will therefore fall in the doppler filter passband, producing appreciable and unusually unacceptable outputs from the filter. This effect frequently places a limit on the degree of doppler filter sidelobe attenuation which can effectively be achieved by current digital signal processing circuits.
The image frequency distortion component is usually measured in magnitude relative to the actual echo signal. A graph depicting the relative magnitude of the image frequency component measured in decibels (db) as a function of amplitude and phase errors representative of the unbalance in the I and Q signal generation channels is shown for a typical baseband processor in FIG. 4. Corresponding to the previous example, it is desirable to maintain the relative image frequency component below -35 db. Therefore, it is apparent from analyzing the graph of FIG. 4 that the baseband processor is very sensitive to these amplitude and phase errors and the image frequency components developed therefrom. It is further apparent from the foregoing discussion that it is of paramount importance to maintain adjustment in the balance between the synchronous detectors and A/D converters in the I and Q signal generation channels, thereby reducing the problems of false alarms or reduction of baseband processor sensitivity as a result of the formation of spurious image frequency component signals.